Regenerators are an integral part of cryocoolers to reach low temperatures between 4 K and 20 K (approximately 270 to 250 K below room temperature) regardless of the refrigeration technique employed; e.g., regardless of whether the known Gifford-McMahon, Stirling, pulse tube, etc. cooling technique is employed. A two stage Gifford-McMahon cycle cryocooler or refrigerator used to reach extremely low temperatures, such as approximately 10 K, without a liquid refrigerant is discussed in U.S. Pat. No. 5,186,765. For discussion of other cryocoolers, see books entitled “Cryogenic Heat Exchangers”, Plenum Press, New York, 1997, by R. A. Ackerman and entitled “Cryocoolers Part 1: Fundamentals”, Plenum Press, New York, 1983, by G. Walker, and the papers entitled “Cryocooler Applications”, Cold Facts, vol. 16, no. 1 (Winter 2000) by R. Radebaugh, pp. 1, 6, 7, 8, 16, 21, 24-25 and “Low-power Cryocooler Survey”, Cryogenics, vol. 42, (2002), by ter Brake and Wiegerinck, pp. 705-718.
One important property of a highly effective regenerator is that the regenerator material should have a large volumetric heat capacity. Most commercial regenerators today employ bronze or stainless steel screens or spheres to cool down to approximately 100 K, and lead (Pb) spheres to cool below 100 K, with 10 K being the no heat-load low temperature limit because the heat capacity of lead becomes extremely low at that temperature. Sometimes a combination of bronze or stainless steel and lead are used for cooling below 50 K with a layered regenerator bed for a single stage refrigerator. Or, a two stage refrigerator is used with a bronze alloy and stainless steel materials used in the high temperature stage and lead (Pb) used in the low temperature stage as a result of the heat capacity of lead not decreasing as quickly as that of the other materials below 100 K. Above 100 K, most metallic, non-magnetic materials have the same molar heat capacity, reaching the DuLong-Petit limit of 3R, where R (=8.314 J/mol K) is the universal gas constant. In general, the higher the heat capacity of the regenerator bed material, the greater the cooling power of a cryocooler, all other parameters being equal.
The potential use of lanthanide intermetallic compounds, which exhibit low magnetic ordering temperatures (e.g. less than 10 K), as cryogenic magnetic regenerator materials (refrigerant or cold accumulating materials) was pointed out nearly 25 years ago by Buschow et al. in an article entitled “Extremely Large Heat Capacities between 4 and 10 K, Cryogenics, vol. 15, (1975), pages 261-264. However, a practical lanthanide regenerator material was not developed and put into use until about 15 years later when the use of Er3Ni (a brittle intermetallic compound) as a low temperature stage regenerator material in a two-stage Gifford-McMahon cryocooler was proposed by Sahashi et al. in “New Magnetic Material R3T System with Extremely Large Heat Capacities Used as Heat Regenerators”, Adv. Cryogenic Eng., vol. 35, (1990), pages 1175-1182 and by Kuriyama et al. in “High Efficient Two-Stage GM Refrigerator with Magnetic Material in Liquid Helium Temperature Region”, Adv. Cryogenic Eng., vol. 35 (1990), pages 1261-1269.
These articles proposed the replacement of the lead (Pb) lower stage regenerator material with Er3Ni intermetallic compound material. Replacement of the lead lower stage regenerator material with Er3Ni material (a brittle intermetallic compound) permitted improved cooling to approximately 4.2 K instead of the approximately 10 K achievable with the previously used lead lower stage regenerator material with a reasonable refrigeration capacity at the lowest temperature. This improvement in cooling (i.e. to approximately 4.2 K) is attributable to the significantly higher heat capacity of Er3Ni than lead below 25 K (the heat capacity of lead becomes negligible below 10 K).
The Gschneidner and Pecharsky U.S. Pat. No. 5,537,826 issued Jul. 23, 1996, describes an improved regenerator for the low temperature stage (e.g. below 20 K) of a two stage Gifford-McMahon cryocooler. The patented regenerator comprises intermetallic compounds Er6Ni2Pb, Er6Ni2(SnxGa1−x), where x is greater than 0 and less than 1, and Er6Ni2Sn as a regenerator component.
An object of the present invention is to reduce the cost and to improve the reliability, efficiency and increase the cooling power of a cryocooler at low temperatures from about 2 K up to approximately 30 K.
Another object of the present invention is to utilize ductile magnetic rare earth (lanthanide) based intermetallic compounds, which can be easily fabricated into tough, non-brittle, corrosion resistant spherical powders, or thin sheets, or thin wires, or screens, or porous monolithic forms (such as cartridges), as the regenerator material.
Another object of the present invention is to provide a cryocooler with a regenerator having significantly higher heat capacity than the aforementioned previously used low temperature (less than 30K) regenerator materials and combinations thereof, such as Er3Ni, HoCu2 and PrxEr1−x. 
More recently, HoCu2 (a brittle intermetallic compound) has replaced Er3Ni as the choice regenerator material for cooling down to approximately 2 K, see Satoh et al., “A Gifford-McMahon Cycle Cryocooler below 2 K”, Cryocoolers 11, R. G. Ross, Jr., editor, Kluwer Academic/Plenum Publishers, New York (2001), pp. 381-386. Also GdAlO3 (a brittle oxide has been suggested as a magnetic regenerator to reach temperatures below that attainable with either Er3Ni and HoCu2, i.e. about 2 K; it orders magnetically at 3.8 K. [Numazawa et al., “New Regenerator Material for Sub-4 K Cryocoolers”, Cryocoolers 11, R. G. Ross, Jr., editor, Kluwer Academic/Plenum Publishers, New York (2001), pp. 465-473].
The low temperature heat capacity properties of several rare earth—copper or silver binary compounds with the CsCl-type crystal, which have magnetic ordering temperatures below 20 K, have been reported in the literatue. However, none of the authors were aware of the ductile nature of these B2, CsCl-type compounds. These include: HoCu, ErCu, TmCu, PrAg, NdAg, (Pr1−xNdx)Ag, TbAg, ErAg, and TmAg. The first measurements were made on HoCu, ErCu, and TmCu, which were found to exhibit two or more magnetic ordering peaks: HoCu—at 13.4, 20 and 26.5 K; ErCu—at 10.9 and 13.8 K; and TmCu—at 6.7 and 7.7 K [“Competition Between Multi-qAntiferromagnetic Structures in Cubic Rare Earth-Copper Compounds”, J. Magn. Magn. Mater., vol. 21, (1980) by Morin and Schmidt, pp. 243-256]. The heat capacities of TbAg and ErAg from 0.5 and 21 K were measured and no magnetic transition was observed below 21 K for TbAg and three peaks at 11, 14.5, and 15.2 K for ErAg [“The Specific Heats of ErAg and TbAg Between 0.5 and 21 K”, J. Phys. F: Met. Phys., vol. 17, (1987) by R. W. Hill]. The heat capacity of ErAg is reasonably large at the 15 K double peak to warrant consideration as a regenerator material. Indeed Japanese scientists have proposed that ErAg be utilized as a regenerator material from 9 to 17 K. [“Evaluation of Low-temperature Specific Heats and Thermal Conductivities of Er—Ag Alloys as Regenerator Materials”, Jpn. J. Appl. Phys., vol. 35, (1996) by Biwa et al., pp. 2244-2248]. The heat capacities of PrAg, NdAg, and (Pr1−xNdx)Ag were measured from 2 to 25 K and only a single magnetic ordering peak was observed. The peak temperatures varied from 10 K for PrAg to 23 K for NdAg, while those for the ternary alloys were 11, 12.5, and 17 K for x=0.1 (also x=0.2), 0.5 (also x=0.6) and 0.8, respectively [“Studies of Low Temperature Specific Heats and Thermal Conductivities of CsCl-type (Pr1−xNdx)Ag (0≦x≦1) Intermetallic Compounds: Application to Regenerator Materials”, Jpn. J. Appl. Phys., vol. 36, (1997) by Yagi et al., pp. 5638-5643]. These authors found that the heat capacity maxima of the ternary alloys are generally significantly less than those of the two end members. They also suggested that PrAg would be a better regenerator alloy than Er3Ni at least over the 8 to 15 K temperature range. More recently, the large heat capacity of TmCu was confirmed, and that of TmAg was reported to be reasonably large at its magnetic ordering temperature, about 8 K [“The Similar Dependence of the Magnetocaloric Effect and Magneto-resistance in TmCu and TmAg Compounds and Its Implications”, J. Phys. Condens. Matter vol. 13, (2001) by Rawat and Das, pp. L379-L387]. This research substantiates the potential of TmCu as a low temperature cryocooler regenerator alloy and suggests that TmAg has only marginal utility as a regenerator material.